1. Field of the Invention
The present invention relates to a functional device having functional elements which perform processing, such as conversion of input signals, alteration of a path, selection of a wavelength and enabling/disabling electrical connection, method of manufacturing for the functional device, and a driver circuit, more particularly, to a functional device which controls the operations of functional elements by a microelectromechanical section.
2. Description of the Related Art
For optical connection network systems, such as an optical fiber system of a wavelength division multiplexing (WDM) type, there are growing needs for the technique that switches optical paths and the technique that selects light of a predetermined wavelength from input light. Such an optical connection network system uses an optical switch at each node on a network, which selectively demultiplexes light of a predetermined wavelength from light with a plurality of wavelengths and then changes the path of the light. The probable future increase in the amount of communication information to be transferred requires the multi-channel and large-scale design of optical devices, such as an optical switch.
Because an optical switch classified into the optical devices changes the path for light without photoelectric conversion of the light, it has characteristics, such as the possible minimizing of the delay time, no dependence on the transfer speed and the expandability. Conventional methods of providing optical switches, which have been proposed so far, include a method which employs the mechanical motion of an optical fiber, a method which is based on the Faraday rotation and a method which uses a reflecting mirror.
Because optical switch which uses a reflecting mirror and employs a microelectromechanical system (MEMS) for the reflecting mirror and a drive apparatus which drives the reflecting mirror is manufactured by using the microfabrication technology to fabricate semiconductor integrated circuits, it is advantageous in cost reduction and large-scale fabrication and is expected as an optical switch which can sufficiently meet the need for the larger scale fabrication of optical switches which will be originated from the future multi-channel design.
For example, Japanese Patent Laid-Open No. 2000-314846 discloses a reflecting mirror formed by an MEMS. Specifically, Japanese Patent Laid-Open No. 2000-314846 discloses a technique of providing a reflecting mirror coupled to a supporting block in a rotatable manner by a beam portion, attaching an electrode to the supporting block and applying a voltage to the electrode so that the operation of the reflecting mirror is controlled by electrostatic force generated between the electrode and the reflecting mirror. Japanese Patent Laid-Open No. 2001-117025 discloses a reflecting mirror formed by an MEMS too. Further, Japanese Patent Laid-Open No. 330254/1999 discloses a technique such that in a semiconductor device equipped with switch means which has a plurality of MOS transistors formed on a substrate and a plurality of switch elements or MEMS formed on the MOS transistors, the switch elements perform switching by moving interconnections provided movably using the Coulomb force. Japanese Patent Laid-Open No. 330254/1999 also describes that with that technique, a variable logic LSI having a higher freedom of design can be realized by providing invariable connection in a semiconductor device with the MOS transistors and providing variable connection with the switch elements.
Japanese Patent Laid-Open No. 144596/1999 discloses a technique of forming an RF switch using an MEMS on a semiconductor monolithic microwave integrated circuit substrate. This technique provides a beam supported rotatably in a seesaw form on the substrate and applying a voltage to an electrode arranged near the beam, thereby generating electrostatic force between the beam and the electrode, which turns the beam. This allows a terminal formed on the substrate to have contact or no contact to a terminal formed on the bottom side of the beam, thereby opening or closing the switch. Japanese Patent Laid-Open No. 144596/1999 describes that the use of the technique can form an array of RF switches with a good sensitivity.
U.S. Pat. No. 5,963,788 (Carole C. Barron, et al.) discloses a technique of preparing a driver circuit which drives MEMS elements on the same silicon substrate on which the MEMS elements are formed.
Japanese Patent Laid-Open No. 2002-36200 discloses a technique of integrating MEMS device modules and an IC control circuit module needed to drive the MEMS device modules on a common systems connecting substrate. This can ensure easy separation of the MEMS device modules and the IC control circuit module for replacement or repair.
The above-described prior arts however have the following problems. While electrostatic force, magnetic force, a piezoelectric effect, thermal expansion and so forth are available as the drive force for a functional element, such as a reflecting mirror or RF switch, a device equipped with such a functional element needs a driver circuit to generate such drive force. In case where electrostatic force is used as the drive force for a reflecting mirror, for example, a driver circuit which selects and controls an MEMS to be driven is needed in addition to an applied voltage generating circuit which generates a voltage.
As shown in FIG. 1 of the aforementioned Japanese Patent Laid-Open No. 2001-117025, for example, such a conventional driver circuit is prepared on a substrate separate from a substrate on which a functional element, such as a reflecting mirror, and a drive apparatus for driving the functional element (hereinafter generally called “MEMS element”) are formed, and is connected to the substrate on which the MEMS element is formed by wire bonding or a flexible substrate or the like. If the scale of an optical device becomes larger due to the multi-channel design and the number of MEMS elements to be driven is increased, the number of interconnections to connect the driver circuit to the individual MEMS elements and the scale of the driver circuit increase, resulting in the enlargement of the overall apparatus. That is, although driving and controlling MESS elements require electrodes, the number of terminals for exchanging drive control signals with an external unit is increased due to the multi-channel design and the large-scaling of the array, thereby increasing the area needed to lay out the associated interconnections. If two electrodes are needed to drive a single MEMS element, for example, a total of 2n2 electrodes are needed for the square matrix layout (array) of n rows by n columns (n being an integer), and terminals equal in number to 2n2 should be provided on the device, which would result in a larger area needed to layout the interconnections to connect to those terminals.
According to the technique disclosed in U.S. Pat. No. 5,963,788, a cavity portion is provided on the top surface of a silicon substrate and an MEMS element is formed in the cavity portion after which a driver circuit is formed in an area in the top surface of the silicon substrate, which is different from the cavity portion. The technique therefore requires a step of protecting the MEMS element at the time of forming the driver circuit and a planarization step after the formation of the driver circuit. This results in an undesirable increase in the number of required steps. In case of laying out an array of several thousand light reflecting mirrors formed by MEMS elements in order to achieve the multi-channel design, the ratio of the area of the cavity portions occupying on the top surface of the silicon substrate increases, thus reducing the mechanical strength of the silicon substrate at the time of preparing the apparatus.
Further, as the technique disclosed in Japanese Patent Laid-Open No. 2002-36200 arranges a plurality of MEMS modules on the systems connecting substrate in a replaceable manner, it is necessary to secure the inter-module alignment precision. In case where each MEMS module includes a mirror for use in optical communications, for example, the optical paths among the modules should be secured precisely. This would result in a complicated assembling process or lower the reliability of assembled apparatuses. Because each MEMS module takes a sealed structure itself, it has a larger volume than an MEMS chip, which makes the apparatus larger in the large-scale array design.